The present invention relates, in general, to semiconductor devices, and more particularly, to resonant tunneling semiconductor devices.
Resonant tunneling diodes and transistors are used in high-performance digital and analog circuits. In the past, resonant tunneling transistors are made by placing a resonant tunneling diode in an emitter contact structure or in the base of a bipolar transistor. Resonant tunneling diodes have two high bandgap barrier layers separated by a quantum well. The quantum well has a ground state energy level E.sub.0 and a first quantized energy level E.sub.1. Outside the two barrier layers, low bandgap material is provided where available charge carriers exist with a Fermi Energy (E.sub.F). Relative energy difference between E.sub.F, E.sub.0 and E.sub.1 is determined by bias applied to the device.
In normal operation, current only flows through the resonant tunneling diode, or through the emitter of a resonant tunneling transistor, by tunneling through the two barrier layers and travelling through the quantum well. Because charge carriers can only travel through the quantum well if they are at an energy approximately equal to E.sub.0 or E.sub.1, the quantum well impedes current flow over a wide range of bias conditions. A "resonant" condition exists, however, when the structure is biased so that E.sub.F =E.sub.0 or E.sub.F =E.sub.1. In these resonant conditions, charge carriers tunnel through the first barrier, travel to the second barrier in the E.sub.0 or E.sub.1 energy level, then tunnel through the second barrier. This feature gives the resonant tunneling diode and transistor a highly non-linear current-voltage (I-V) characteristic with negative differential resistance finding utility in high frequency oscillators and both analog and digital circuits.
The high current density which flows in the resonant tunneling device during the resonant condition is called peak current. The relatively low current density which flows through the device in the non-resonant condition is called valley current. One problem with prior resonant tunneling devices is that the ratio of the peak current to the valley current is not as large as is desired for efficient circuits. To provide high current density devices, peak current should be large. This can be done by lowering the bandgap of the barrier layers, but this method results in larger valley current because the barriers do not impede current as well during the non-resonant condition. The large valley current, which is analogous to a leakage current in a conventional transistor or diode, is dissipated as heat in the device. Devices with large valley current are thus inefficient and create power dissipation and heat removal problems for circuit designers.
Another problem with existing resonant tunneling devices is that large peak-to-valley ratios are needed to provide a large negative differential conductance to obtain maximum frequency of oscillation. Large peak-to-valley ratios are also needed to provide reliable switching for analog and digital circuits.
What is needed is a resonant tunneling semiconductor device that allows for large peak currents while at the same time providing low valley currents.